A tuner is provided in receivers such as radios and televisions to tune to a channel selected by a user. The tuner typically comprises a mixer that mixes the received RF signal with a local oscillator signal having a frequency corresponding to a selected channel. The output of the mixer is a predetermined intermediate frequency signal. In order to prevent the mixer from being overloaded by the received RF signal, an attenuator, such as a PIN diode attenuator, is provided between the RF input and the mixer. The attenuator is intended to ensure that the signal in the tuner is not so large so as to overload the mixer.
FIG. 1 shows an upconverting section 10 of a double conversion tuner without band limiting that provides an example of attenuation control in accordance with the prior art. The upconverting section 10 has an RF input 12 that receives an RF signal such as from an antenna. The RF signal is supplied to an attenuator 14 (such as a PIN diode attenuator) that is provided to attenuate the RF signal, and the RF signal at the output of the attenuator 14 is amplified by an RF amplifier 16. The RF signal at the output of the RF amplifier 16 is supplied through a directional coupler 18 to a broadband filter 20 that is arranged to filter out signal components having frequencies outside of a selected range. For example, for television applications, the broadband filter 20 filters out signal components having frequencies outside of the frequency range of 50-800 MHz. The RF signal at the output of the broadband filter 20 is provided to the RF input of a mixer 22 which also receives a local oscillator signal LO from a local oscillator. The mixer 22 mixes the RF signal on its RF input with the local oscillator signal to produce an output IF signal.
The directional coupler 18 provides coupling to the incident voltage on the line between the RF amplifier 16 and the broadband filter 20 to an amplifier 24, and this amplified voltage is provided to a detector 26. In this way, the detector input level is not affected by input impedance variations of the broadband filter 20 which otherwise would cause attenuation variations. The detected signal at the output of the detector 26 is used to control the attenuator 14 such that, as the incident voltage at the output of the RF amplifier 16 becomes too large, the RF signal is attenuated to prevent overloading of the mixer 22.
The problem with the upconverting section 10 is the cost of the directional coupler 18 and the technological incompatibility of the directional coupler 18 with the eventual tuner integration process.